This week saw a number of developments at Twitter and Facebook, two of the world’s biggest social networking sites, that are signs of them looking to capitalize more on their already-hefty user bases by creating services that keep them on their sites for longer.

Twitter bought up Summify, a news summary service, and Facebook introduced some 60 new apps to its network. And Facebook could soon be extending time spent on the site even more: a report out today says that Facebook is in talks to replace YouTube (NSDQ: GOOG) as the host for premium music video service Vevo.

The report, from CNET, says that Facebook has held talks with Vevo to move its video service over to Facebook when its contract with Google’s YouTube expires in a year. The report cites sources close to the matter, but has not confirmation from any of the named parties.

The arrangement, CNET writes, would be similar to what Vevo already does with Google on YouTube: Facebook would host and stream the videos, and Vevo and Facebook would both sell and share revenues for ads that run alongside those videos.

Vevo could be a lucrative partner for Facebook in its bid to grow advertising on the site. According to November 2011 figures from Nielsen, Vevo is second to YouTube in terms of unique users (43 million versus 130 million). Facebook itself has made a lot of moves to add video partners to its service too, and it comes in fourth at 30 million. When it comes to the total number of streams, however, Vevo pales to YouTube (535,000 vs 13.4 million), and Facebook does not rank at all. Ditto on time spent per user, where neither Vevo nor Facebook make the top ten, and YouTube is in fourth place (Netflix (NSDQ: NFLX) is the king of time spent per user, by the way).

A deal with Vevo would also follow a pattern of moves that Facebook is making to increase user engagement.

The company this week also introduced a lineup of new apps — some 60 of them — specifically designed for users to add into their Timeline, the new graphical homepage view that Facebook has introduced.

These apps will add more functionality — and hence, the hope is, engagement — in its new Timeline feature. They cover all manner of special interests, from food (a Foodspotting app) to crafts (a Pinterest app) and travel (TripAdvisor) and of course those social games that are a cornerstone of Facebook’s regular apps catalog. Facebook says that the list will grow over time.

Timeline has seen some controversy — some complain they don’t like the look of it; others question how Facebook will use the new layout and information manipulation for commercial gain. In that sense, the apps will either win skeptics over with their usefulness, or make them even more suspicious of how Facebook is encouraging information sharing, even when it does provide some options for limiting that.

When you think about it, it’s not too surprising to see Facebook adding more usefulness and stickiness to its site, as it seeks to grow its primary form of making money: advertising. Eyeballs are the currency of online advertising and so social networks have to figure out not just how to grow their user bases, but how to keep them interested once they are there. That’s an area where Twitter is also focusing:

Yesterday, it was announced that it would be acquiring Summify, a Vancouver-based startup that developed a way to read users’ feeds from Twitter, Google Reader and Facebook and pull out relevant news into a summarized list. The selections, which can be viewed via an app or the web, are made based on your own browsing history, as well as the popularity of particular stories (eg, those that have been retweeted a lot).

Twitter has not revealed much about the deal: no details of the price, or how, exactly, Summify will work as part of Twitter. From a blog post at Summify, we do know that the company has stopped taking new users and will evenutually shut down the standalone service. Also, its employees are packing their bags and moving to Twitter’s HQ in San Francisco.

But given that Twitter, too, is looking to grow engagement on its site — a recent hire in the UK from BBC’s Sports division also demonstrated that — you can see where a service like Summify could potentially fit in: a secondary view of your followers, perhaps, maybe integrated to have a better experience around a user’s lists and favorited tweets. The added information a curated list of news would provide on content consumption could also come in handy for figuring out what ads to point a users’ way, too.